1. Field of the Invention
The invention relates to an impedance matching circuit, and more particularly, to an impedance matching circuit without external resistor and capable of automatically adjusting the impedance.
2. Description of the Prior Art
Impedance matching is one of the major considerations of high-speed data transmission. Once the impedance value of an input terminal more closes to a transmission medium, the reflection power will be reduced, and thus the input terminal can obtain a better signal quality. In general, impedance matching techniques are applied in transmission interfaces of analog signals such as LCD display controllers, network controller chips, etc. When the impedance value of the transmission medium is 50 ohms or 75 ohms, the input terminal will adjust the impedance value as close as possible to 50 ohms or 75 ohms respectively.
In general, the concept of the impedance matching circuit is to add a precision resistor outside an IC for adjusting the equivalent impedance inside the IC to become close to the impedance value of the precision resistor. Since the impedance value of the external precision resistor is equal to the equivalent impedance of the transmission medium, the impedance matching purpose can be attained at the input terminal. A disadvantage of this conventional method, however, is that at least a pin is required to be connected to the precision resistor, and therefore package volume and fabrication cost of the IC will be increased.